In mobile communications, there is an increasing demand for higher system capacity and end-user data rates. Data rates of the order of 10 Gigabits per second (Gbps) can be practically achieved only with a sufficiently large transmission bandwidth, significantly larger than the current maximum of 100 Megahertz (MHz) for the Long Term Evolution (LTE) standard. Such demands for very high system capacity and very high end-user date rates can be met by so-called Ultra-Dense Networks (UDNs). UDNs may be regarded as networks with access-node densities considerably higher than the densest cellular networks of today. UDNs may be set up with distances between access nodes from a few meters (m) in indoor deployments up to around 50 m in outdoor deployment.
UDNs may be expected to use a maximum transmission bandwidth of up to around 1 to 2 Gigahertz (GHz). Such very wide transmission bandwidths are realistically only possible at higher frequency bands beyond 10 GHz. For example, frequencies in the lower part of the millimeter wave band (mmW) up to 100 GHz may be of specific interest for UDNs. For this purpose, communication systems are densified more and more by providing a higher number of access nodes with smaller distances (measured from one access node to another access node) as compared with common communication systems. A network operating at mmW frequencies may require a dense mesh of network nodes given propagation conditions (high path loss) at such high frequencies. In such dense environments, it is not always possible to provide a wired backhaul to each access node. Thus, a solution where access nodes can be wirelessly backhauled by other access nodes becomes an attractive complement.
In the UDN context wireless self-backhaul for a set of UDN nodes and interference aware routing solutions for routing packets through the backhaul networks have been proposed by D. Hui and J. Axnäs in the paper “Joint Routing and Resource Allocation for Wireless Self-Backhaul in an Indoor Ultra-Dense Network”, PIMRC 2013. With self-backhauling, an access node serves not only its own assigned User Equipments (UEs) in the vicinity but also its neighboring access nodes as a relaying node in order to route data towards and/or from an information aggregation node. To maximize the throughput of each route, a route selection process takes into account the mutual interference among wireless links. According to the concept of the aforementioned paper, one approach is to jointly optimize route selection and radio resource allocation. For this purpose, the original network may be transformed to an expanded virtual network in which each virtual node represents a possible way of allocating radio resources to the access node. A route selected in such a virtual network jointly determines a sequence of access nodes (i.e. the real route) and the corresponding radio resources allocated to the links associated with these nodes.
This and similar concepts provide a solution focusing on interference aware routing under full buffer assumptions.
A route is then the end-to-end path from an aggregation node (access node with wired backhaul) to the desired destination node, e.g. a User Equipment (UE). The individual hops in this end-to-end path are denoted links.
The routing—i.e., the process of finding a route or path from a source node to the destination node—is done often together with a (crude) resource allocation since interference between wireless links makes routing and resource allocation a dependent problem. Routing is a rather slow process since channel state information from throughout the network needs to be available for making routing decisions. Therefore also the resource allocation to links performed during routing is slow. The long-term assignment of resources to (a group of) link(s) may include at least dedicated (which can be referred to as green resources), prohibited resources (which can be referred to as red resources), and shared resources (which can be referred to as yellow resources). Dedicated resources can always be used by a link as there is no danger of interfering severely with other links. Prohibited resources are not allowed to be used by a link. Shared resources may interfere with other links and measures need to be taken to mitigate potentially high interference.
As stated above, the assignment of resources during routing is slow. The resources actually used for a transmission is determined by the Medium Access Control (MAC) layer. This is the fast resource allocation process. If a link has sufficient dedicated resources assigned to it these resources will be used first. If the dedicated resources do not suffice then also shared resources will be used. Since shared resources are not guaranteed to avoid heavy interference with transmissions on other links the usage of shared resources must be disseminated throughout the network to inform other nodes that these shared resources are used and other nodes potentially sustain from using them.
In unpublished patent application PCT/EP2014/051131 of the applicant flooding is described as a solution of how to disseminate resource information throughout the network. Flooding may be summarized shortly as follows. Flooding uses the already established links in the network. First, a node (the source) transmits a resource reservation (a resource reservation may be understood as a wish for a resource allocation, which may fail or not) by flooding to all other nodes in the network. Then, other nodes may refrain from transmitting in resources that they know would interfere with the claimed resource. If it is not interfering the resource may be spatially reused by other links.
Typically it is not sufficient to reserve resources for a single link only. Typically, resources will be needed on multiple links of the route. With the current solution one resource reservation process having the same resource reservation message(s) flooded throughout the network is needed for each link.